Method of heating metallic material

ABSTRACT

In a continuous method of heating metallic material the surface of which tends to react chemically with the surrounding atmosphere when subjected to heat, the metallic material, for the purpose of avoiding such reaction, is charged to a generally gas tight furnace, while substantially free from oxidic coatings. The furnace is heated in a manner such as to exclude the burning of fuel in the actual furnace space. The quantity of gas capable of reacting with the material and entering the furnace space as the furnace is charged and discharged is restricted, so as to retain a balance between the furnace atmosphere and the material.

Hamrin et al.

METHOD OF HEATING METALLIC MATERIAL Inventors: Bo Goran Hamrin, Fagersta; Rune Peterson, Vasteras, both of Sweden Assignee: Granges Engineering Aktiebolag,

Vasteras, Sweden Filed: Feb. 27. 1973 Appl. No.: 336,353

Foreign Application Priority Data Feb. 29, 1972 Sweden i. 2543/72 US. Cl. 432/23; 432/26; 432/152;

432/198 Int, Cl. F27b 5/04; F27b 9/58 Field of Search 432/23, 26, 19, 152, 198

References Cited UNITED STATES PATENTS 133 2/1940 Pearson 432/23 31 29 32 3 [h/Ehjg 12 f 1 Aug. 12, 1975 2,253,897 8/1941 Doderer 432/23 2,421,482 6/1947 Crocker 432/23 2,749,106 6/1956 Ness 432/23 Primary Examiner-John J. Camby Attorney, Agent, or Firm-Waters, Schwartz & Nissen 5 7 ABSTRACT ln a continuous method of heating metallic material the surface of which tends to react chemically with the surrounding atmosphere when subjected to heat, the metallic material, for the purpose of avoiding such reaction, is charged to a generally gas tight furnace, while substantially free from oxidic coatings. The furnace is heated in a manner such as to exclude the burning of fuel in the actual furnace space. The quantity of gas capable of reacting with the material and entering the furnace space as the furnace is charged and discharged is restricted, so as to retain a balance between the furnace atmosphere and the material.

2 Claims, 2 Drawing Figures METHOD OF HEATING METALLIC MATERIAL The present invention relates to a continuous method of heating metallic material the surfaces of which tends to react chemically with the furnace atmosphere when subjected to heat, in a manner such as to substantially avoid any such reaction, in which method the metallic material is heated in a furnace into which it is charged while substantially free from an oxygen-containing coatings. The invention is primarily intended for the heat treatment and heating of steel ingots, billets and the like, particularly ingots and billets comprising high grade steel and containing large quantities of alloying substances, and in the following will mainly be described with reference there to. It should be understood, however, that the invention can also be applied to advantage in the heat treatment and heating of other metals which hitherto have the tendency of deleteriously reacting with or of being affected by the atmosphere in which the heating process or heat treatment process is carried out.

Decarbonization and scaling, i.e. the depletion of carbon at the surfaces of ingots, billets, blooms etc. and the oxidation of the iron in the surface portions thereof when heating the ingots, billets, blooms etc. to hot working temperatures have always been considered practically unavoidable occurrences. Decarbonization and scaling result in large quantities of waste, especially in the manufacture of highly alloyed high-grade steel, such as high-speed steel, tool steel, stainless steel etc. Because of decarbonization, it is often necessary in practice to form the ingots, billets, blooms etc. with large working tolerances, so that the decarbonized surface portions thereof can be removed by conventional machining operations such as scraping, grinding, scale turning etc. Although it is often possible to recover the material removed to some extent, by using the same as scrap charge in steel manufacturing processes, decarbonization nevertheless results in an economically serious yield loss, while the costs incurred by the necessity of removing the decarbonized portions are burdensome. The metal present in the scale formed on the in gots, billets, etc. as a result of scaling when heating same to the hot working temperature cannot often be recovered in an economic manner, the scale normally falling from the surface of the ingots, billets, etc. into the furnaces, rolling mills etc. used in the treatment of the heated metal. The material losses caused by scaling can, in the case of highly alloyed, high-grade steels, be very significant from the aspect of economy, and costs are often increased owing to the fact that because of scaling it is often necessary to pickle the heated ingots, billets, etc. before they can be processed further.

A number of methods of avoiding decarbonization and/or scaling have been propsed during recent years. All of these methods, however, have been encumbered with disadvantages of a practical and/or an economic nature, which has limited their usefulness within the industry.

The object of the invention is to provide a new and improved method in the heating of metallic material whose surface tend to react chemically with the furnace atmosphere, by which method undesirable surface reactions such as decarbonization and scaling are avoided to a high degree in a practical and economic manner. To this end, it is proposed in accordance with the invention that when practicing a method of the type defined in the introduction, the material is heated in a substantially fully gas-tight furnace, which is heated without burning fuel in the actual furnace space itself and in an atmosphere which is generally in balance with the composition of the material being heated at the heating temperature in question and substantially all of which is constantly retained in said furnace space, and by restricting the quantity of gas capable of reacting chemically with the material and entering the furnace as said material is fed into and out of the same in a manner so as to maintain the balance between the furnace atmosphere and the metallic material at such a level that only a slight, controlled degree of chemical reaction takes place between the metallic material and the gas in the furnace space. Thus, the method of the present invention uses the actual material being heated to form a suitable furnace atmosphere. This is done by causing the surface layers of the metallic material to react with the gas which enters the furnace space when the material is fed into and out of the furnace and the gas which it may be necessary to charge to the furnace space in order to maintain a suitable pressure above atmospheric therein. The quantity of gas entering the furnace space incidently and optionally fed there to in order to maintain said pressure is held so low, by retaining in the furnace to the highest possible extent the gas already present therein, that the surface of the metallic material is not affected to an extent which causes the surface quality of the material to depart from the requirement placed thereon.

Normally, the metallic material is fed in batches into and out of the furnace through at least one feed valve or lock, which is capable of being closed to the furnace space and to the surrounding atmosphere and the cubic capacity of which suitably only slightly exceeds the volume of the batch. In this respect, for the purpose of removing harmful quantities of oxygen from feed valves containing the batch, the feed valve, before being placed in communication with the furnace space and while closed against the surrounding atmosphere, at least when discharging the material from the furnace and suitably also when feeding the same thereinto, can be flushed with a gas which is at least substantially inert with 'respect to the metallic material, or may also be first subjected to subpressure and then flushed with a gas which is at least substantially inert with respect to the metallic material.

In accordance with an alternative embodiment of the method according to the invention, the difference in the cubic capacity of the feed valve and the volume of the metallic material fed thereinto for charging to the furnace is maintained at such a small magnitude that the quantity of air present in the feed valve together with the material during a furnace charging operation is insufficient to react with the surface of the metal to a harmful extent, thereby rendering it unnecessary to remove air from the feed valve by flushing or to establish a subpressure therein in accordance with the aforegoing.

To eliminate all risk of unintentional leakage of air into the furnace space, the pressure within the furnace space should be slightly higher than the pressure of the atmosphere surrounding the furnace. In accordance with the invention, the pressure in the furnace space is maintained at the desired level by feeding into or out of the same minor quantities of a gas which is at least substantially inert to the material in question.

The invention will now be described in more detail with reference to the accompanying drawing, which diagrammatically illustrates by way of example only a furnace plant selected for carrying out the new method. Thus,

FIG. 1 is a vertical sectional view taken through the line I-I in FIG. 2, and

FIG. 2 is a sectional view of the furnace plant taken through the line II-II in FIG. 1.

In the drawing, the reference numeral generally indicates a furnace which is substantially completely sealed against the surroundings and which is intended for heating rod-shaped workpieces such as ingots or bi]- lets etc. (not shown) and to which is connected an infeed valve or lock 11 and an outfeed valve or lock 12. The furnace 10 is provided with a number of electric heating elements 13 which extend through the furnace roof 14 down into the furnace space 15. A shield 16 comprising walls and roof prevents unintentional access to the portions of the elements 13 located above the furnace roof and having connections (not shown) for conducting electric current to the elements. Other methods of heating the furnace 10 can be applied within the scope of the invention, provided that the furnace is heated without combusting fuel in the actual furnace space itself. The feed valves 11, 12, which are elongate in form and extend substantially parallel with each other, open into an end wall 17 of the furnace 10 adjacent their respective furnace side walls 18, 19. Arranged in the furnace are two roller paths comprising rollers 20 and 21 respectively, each path extending along its respective one of the walls 18, 19, and beams 22 and pusher means 23 for supporting and feeding the workpieces entering the feed valve 11 transversely through the furnace space 15 from the rollers 20 towards the rollers 21. The rollers 20 and 21 are carried by shafts which are journalled externally of the furnace 10 in bearings 24 carried by bracket structures 25. The pusher means 23 have the form of plates connected to the pistons of respective linear piston engines 27, via pusher rods 26 passing through the furnace wall 18, the stroke of which engines is at least such that subsequent to a workpiece resting on the rollers 20 being moved towards the rollers 21 there is provided the requisite space for feeding a new workpiece into the furnace space 15 from the feed valve 11. The piston engines are placed on bracket structures 28. Sealing means (not shown) are arranged to seal between on one hand the roller shafts and the rods 26 and on the other the furnace walls 18, 19.

The feed valves 11 and 12 are provided at each end thereof with closure means, as indicated at 29. The ref erence numerals 30 and 31 identify rollers forming part of diagrammatically illustrated roller paths for feeding workpieces into the feed valve 11 and discharging heated or heat-treated workpieces from the feed valve 12. The feed valves 11, 12 are also provided with feeding mechanisms 32 for feeding the workpieces from the valve 11 to the roller paths formed by rollers 20 in the furnace 10 and for feeding the heated or heat-treated workpieces from the valve 12 to the roller paths formed by the rollers 31. To each of the feed valves 11, 12 there is connected a pipe stud 33, 34 or a pair of such studs, via which the air in the feed valves can be removed by suction and/or the feed valves can be flushed with a substantially inert gas. When the feed valves 11, 12 are to be flushed, the flushing gas can be introduced through the pipe studs 33 while the feed valves are in communication with the surrounding atmosphere, wherewith the stud pipes 34 can be omitted.

For the purpose of maintaining a suitable pressure in the furnace 10, there is arranged a line (not shown) through which the gas, which is substantially inert with respect to the workpieces being treated, can be charged to the furnace should the pressure in the furnace fall below a predetermined level. The pressure in the furnace 10 is suitably selected so as to slightly exceed the pressure of the surrounding atmosphere. The gas in the furnace space, and also the gas used for flushing the feed valves 1 1, 12, may comprise without detriment an inexpensive so-called inert gas, e.g. commercially available nitrogen gas, which contains a certain quantity of oxygen since the gas charge used for flushing the feed valves 11, 12 does not have to pass through the furnace.

A suitable atmosphere lacking any appreciable tendency to oxidize or to decarbonize the surface of the workpieces when heating same to the relevant temperature may be initially created by filling the furnace 10 with a substantially inert gas whose carbon and oxygen potential is subsequently adjusted so as to be substantially in balance with the composition of the material from which the workpieces are formed by allowing the gas to react with the workpieces first charged to the furnace.

Since the workpieces first charged to the furnace may therefore be oxidized or decarbonized to an appreciable extent, it is suitable to initially charge the furnace with second grade workpieces or suitably with scrap material free from oxidic surface coatings. It is also possible to initially fill the furnace with air and to create a suitable furnace atmosphere solely by heating, for example, suitable scrap material in the furnace. When a suitable furnace atmosphere whose carbon and oxygen potential at the heating temperature in question is in balance with the composition of the goods to be heated is reached in the furnace, the actual heating or heat-treatment process of the workpieces is commenced, the furnace gas adjusted with respect to its compositions being retained in the furnace to the highest possible degree and supplementary gas being charged to the furnace 10 or allowed to enter thereinto only to the extent necessary for maintaining a suitable pressure in the furnace. To restrict the quantity of gas entering the furnace as workpieces are fed thereinto or discharged therefrom, the air is sucked and/or flushed from the feed valves 1 1, 12 while communication of the valves with the furnace is closed, the feed valves being constructed to advantage so that their cubic capacity only slightly exceeds the volume of the charge being fed into and out of the furnace, thereby considerably reducing the quantity of flushing gas required. With such small differences in volume between the cubic capacity of the feed valves and the volume of the charge as those envisaged here, it is often possible, when the feed valves are emptied of air by subjecting the same to a sub-pressure, to replace the air removed by suction with gas from the furnace space 15 as the charge is fed into and out of the furnace, and in certain instances it is not necessary to flush the feed valves when charging material to the furnace.

The advantages afforded by the new method will now be illustrated with reference to two examples.

EXAMPLE 1 A high-alloy, high speed steel material having a specific surface of 1650 dm was fed each hour through a conventional furnace having an infeed lock and an out feed lock. Each lock had a capacity of 18 litres and the lock charging and discharging frequency was one batch of material per minute. As is conventional, each time i batch was fed into the infeed lock, a quantity of gas equal to times the lock capacity, i.e. 180 l of gas, was fed to the infeed lock through the furnace to displace gas present in the lock to atmosphere and to flush the batch of material in the lock. Similarly, the outfeed lock was flushed in a corresponding manner, with 180 litres of flushing gas, prior to discharging a batch of material from the furnace. The gas used was N with 1.5 by volume 0 Thus, with each hour 21.600 Nl of gas were charged to the furnace, the total charge of oxygen reaching to approximately 325 Nl/hr. It can be readily calculated from this that 0.2 N1 of 0 was charged for each dm of material, which was far in excess of the level of 0.04 Nl per dm permitted in the case of the material in question.

EXAMPLE 2 The same furnace as that described in Example 1 was used under the same conditions to heat with each hour a similar material having a specific surface of 1650 dm although in this instance the locks were flushed with gas fed directly to the locks, i.e. without passing the gas through the furnace. Thus, in distinction to the conventional method described in Example 1, only that quantity of gas corresponding to the capacity of the feed lock enters the furnace space, this volume of gas being 2.160 Nl/hr of the same gas. The amount of oxygen charged was thus 0.02 N] per dm of material per hour. It was necessary to supply 250 N] of the above gas per hour to maintain the pressure in the furnace space. This represented an oxygen charge of 3.75 N], i.e. an addition of about 0.002 N1 0 per hr and dm which was well below the limit permitted for the material in question.

The invention is not restricted to the described and illustrated embodiment. Thus, the furnace may comprise a walking beam furnace, a pusher type furnace or any other suitable furnace, and when an annular furnace is used in conjunction with the method of the present invention the same feed valve can be used for feeding a charge into and out of the furnace. Thus, the method of the present invention can be modified in a number of ways within the scope of the following claims:

What is claimed is:

l. A continuous method of heating metallic material in a substantially gas-tight furnace in a manner to substantially avoid changes in the chemical composition of the surface portions of the material, said gas-tight furnace being provided with at least one feed valve, which is held closed against the surrounding atmosphere when placed in communication with the furnace space and which is held closed against the furnace space when placed in communication with the surrounding atmosphere, for feeding the material batchwise into and out of the furnace, the method comprising the steps of:

a. charging the material substantially free from oxygen-containing coatings into the furnace;

b. heating the material in said furnace in the absence of burning fuel within the actual furnace space;

c. maintaining an atmosphere in said furnace space which is substantially in a chemical balance with the composition of the material being heated at the relevant heating temperature by constantly retaining substantially all of the gas forming said atmosphere in the furnace space and by allowing gas capable of reacting chemically with the material to enter the furnace space as said material is fed into and out of the furnace in restricted quantities insufficient to destroy said balance;

d. flushing the feed valve with a gas which is substantially inert with respect to said material at least before discharging a material batch from the furnace;

e. subjecting the feed valve to a subpressure at least before discharging a material batch; and

f. using material batches each having a volume which is only slightly smaller than the cubic capacity of the feed valve.

2. A method according to claim 1, maintaining the pressure in the furnace space at a predetermined level by introducing into the furnace space and removing therefrom small quantities of gas, the gas introduced being at least substantially inert with respect to said material. 

1. A continuous method of heating metallic material in a substantially gas-tight furnace in a manner to substantially avoid changes in the chemical composition of the surface portions of the material, said gas-tight furnace being provided with at least one feed valve, which is held closed against the surrounding atmosphere when placed in communication with the furnace space and which is held closed against the furnace space when placed in communication with the surrounding atmosphere, for feeding the material batchwise into and out of the furnace, the method comprising the steps of: a. charging the material substantially free from oxygencontaining coatings into the furnace; b. heating the material in said furnace in the absence of burning fuel within the actual furnace space; c. maintaining an atmosphere in said furnace space which is substantially in a chemical balance with the composition of the material being heated at the relevant heating temperature by constantly retaining substantially all of the gas forming said atmosphere in the furnace space and by allowing gas capable of reacting chemically with the material to enter the furnace space as said material is fed into and out of the furnace in restricted quantities insufficient to destroy said balance; d. flushing the feed valve with a gas which is substantially inert with respect to said material at least before discharging a material batch from the furnace; e. subjecting the feed valve to a subpressure at least before discharging a material batch; and f. using material batches each having a volume which is only slightly smaller than the cubic capacity of the feed valve.
 2. A method according to claim 1, maintaining the pressure in the furnace space at a predetermined level by introducing into the furnace space and removing therefrom small quantities of gas, the gas introduced being at least substantially inert with respect to said material. 